The present invention relates to the field of security and fire protection. More particularly, it relates to improvements in security/fire alarm systems in which a relatively large number of remote sensors are actively monitored by a central control unit to ascertain their respective alarm and/or operating status.
Sophisticated security and fire alarm systems typically include a large number of various types of remote "event-sensors" for detecting, in a variety of ways, unauthorized entry, robbery attempts, fire, etc. To verify that each of the sensors is, indeed, operational and is, or is not, reporting an alarm condition, it is common in the art to provide a multiplex communication system by which a central control unit interrogates the sensors. In such a system, each of the sensors is assigned a unique address code and is linked to the control unit via a communications bus. The control unit interrogates each sensor by transmitting its respective address code on the bus and monitoring the bus for a response transmission by the addressed sensor. While such transmission and response typically takes on the order of milliseconds (e.g., 20-30 msec.) to complete for each sensor, it will be appreciated that a cycle time of the order of seconds can be required to complete the interrogation of 100 sensors. In many applications, such a cycle time is more than adequate for reporting a sensor failure or an alarm condition. But there are applications in which only a few seconds delay can spell the difference between success and failure of the system. Consider, for example, a situation in which a "hold-up" button is pressed and, due to the number of sensors on the bus, several seconds are required to activate a CCTV (closed-circuit TV). During such interval, the would-be robber may, for example, turn away from the camera and thereby conceal his identity.